1. Technical Field
The present invention generally relates to lubricating base oil compositions containing at least one Fischer-Tropsch derived lubricating base oil and at least one polyol ester and to methods for improving fuel economy in internal combustion engines using same.
2. Description of the Related Art
There is currently a need to improve the efficiency and useful life of engine oils, particularly those used as crankcase lubricants in internal combustion engines in such vehicles as, for example, automobiles and trucks. Generally, engine oils used in internal combustion engines have a limited life. Their efficacy can be curtailed by such factors as, for example, the expiration of the specific additive package that is blended into the base oil(s), breakdown of the oil in the engine due to oxidation, etc.
The viscosity grade of an engine oil is one feature when selecting a lubricating oil. The lubricating oil is typically chosen according to both the climatic temperatures to which the engine is exposed, and the temperatures and shear conditions under which the engine operates. Thus, the oil must be of sufficiently low viscosity at ambient temperatures to provide adequate lubrication upon cold starting of the engine, and capable of maintaining sufficient viscosity to lubricate the engine when it is under a full operating load.
The Society of Automotive Engineers classification system, SAE J300, defines engine oil grade viscosity specifications. Single grades are designated as SAE 20, 30, 40, 50, and 60 grade, and are defined by a low shear rate kinematic viscosity range at 100° C. (ASTM D445), as well as a minimum high shear rate viscosity at 150° C. (such as ASTM D4683, CEC L-36-A-90, or ASTM D5481). Engine oils designated as SAE 0W through 25W have been classified according to their low temperature cranking viscosities (ASTM D5293), low temperature pumping viscosities (ASTM 4684), and a minimum kinematic viscosity at 100° C. Multigrade oils meet both the high and low temperature viscosity requirements indicated in their nomenclature. For example, an engine oil designated as SAE 5W-30 possesses the viscometric characteristics of SAE 30 motor oils as well as the low temperature viscometric qualities of SAE 5W.
A lubricating oil should be chosen with the appropriate high temperature kinematic and high shear rate viscosities for a given engine type and operating conditions in an effort to prevent the engine wear and oil consumption that can be associated with inadequate boundary layer lubrication and oil thinning, respectively. Similarly, to afford low temperature engine protection, the maximum low temperature cranking and pumping viscosities of the lubricant should match the requirements imposed by the environment in which the engine will be operated. The maximum low temperature viscosity limits of a given oil grade are intended to define the oil's ability to facilitate engine starting in cold weather, and to ensure the ready flow of cold oil to the oil pump, thereby minimizing the potential of engine damage due to insufficient lubrication.
In addition to selecting the appropriate multigrade oil, it is also necessary for the internal combustion engine to have its oil changed periodically in order to maintain the efficiency and mechanical integrity of the engine. However, it is common for oil change intervals to lag behind other service intervals since consumers can view oil changes as one of the more inconvenient and, in some cases, costly regular maintenance aspects of vehicle ownership. Typically, recommended oil service intervals have been extended with the introduction of higher quality base stocks and better lubricant additive packages. Yet regular oil changes still tend to lag behind other maintenance items such as, for example, air filter replacement, brake replacement, etc.
There are also a number of situations in which there is a desire to utilize an engine oil having a prolonged service life. For example, to minimize maintenance costs and equipment downtime, leasing companies and equipment operators such as trucking companies desire engine oils that retain their original performance capabilities for prolonged periods of service. Additionally, original equipment manufacturers desire lubricating oil compositions having extended drain capabilities whereby their lease customers can operate the equipment for longer periods of time or for greater distances before draining the engine oil and replacing all or part of it with a new engine oil.
There has also been an increasing concern in recent years to improve the fuel economy performance of an internal combustion engine, particularly that of passenger car engines and diesel fuel engines. Fuel economy in the United States is federally mandated by Corporate Average Fuel Economy (CAFÉ), specifying the fleet-average fuel economy for passenger cars. Improved fuel economy can result in more environmentally desirable compositions, the use of less fuel per mile traveled, hence lower greenhouse gas emissions and also conservation of petroleum, an increasingly scarce resource. The viscosity of the engine oil is one factor that influences fuel economy. The lower the oil's viscosity the lower the viscous drag on the engine and hence the better the fuel economy performance. However, the lower viscosity grade oils must still provide adequate lubrication to protect the engine.
Accordingly, there remains a need for lubricating base oil compositions having an extended life period while also providing significantly improved fuel economy.